Multi-level optical recording medium reproducing method and reproducing device

ABSTRACT

A reproduction method for a multi-level optical recording medium according to the present invention rotates an optical recording medium, on which recording data has been recorded according to a multi-level recording method which sets several levels for light reflectivity of virtual recording cells, emits a reproduction laser beam towards the optical recording medium, and reproduces the recording data based on an electric signal generated in accordance with a received level of reflected light, the method setting the emission power of the reproduction laser beam in a range of 1.0 mW to 2.5 mW inclusive when the optical recording medium is rotated at a linear velocity in a range of 9 m/s to 25 m/s inclusive. By doing so, it is possible to improve the read accuracy for recording data while suppressing the reproduction deterioration of the optical recording medium to a level where deterioration effectively does not occur.

TECHNICAL FIELD

[0001] The present invention relates to a reproduction method for amulti-level optical recording medium on which recording data has beenrecorded according to a multi-level recording method and a reproductionapparatus for reproducing recording data according to the reproductionmethod.

BACKGROUND ART

[0002] At present, as optical recording media, binary optical recordingmedia constructed so that binary data can be recorded by forming pits byirradiation with a recording laser beam and the binary data can bereproduced based on the presence/absence of such pits are in widespreaduse. Due to demands in recent years for improvements to the recordingdensity of optical recording media, research has been conducted into arecording method that adjusts the focused beam diameter of a recordinglaser beam to record recording data at a high density.

[0003] On the other hand, aside from methods that adjust the focusedbeam diameter, the development of multi-level optical recording mediawhere one out of a plurality of marks with different meanings can berecorded in each virtual recording cell is also progressing. With suchmulti-level optical recording media, a characteristic is used whereby,for example, an altered part (hereinafter also referred to as a“recording mark”) where the light transmissivity is lowered appears inone part of a virtual recording cell that is being recorded upon on theoptical recording medium due to an irradiated amount of the recordinglaser beam being switched between several levels, and the proportion ofan entire virtual recording cell taken up by a recording mark variesaccording to the irradiated amount of the recording laser beam. That is,when a reproduction laser beam is irradiated onto this multi-leveloptical recording medium, an effect is caused by the lighttransmissivity in virtual recording cells in which the above-describedrecording marks have been formed, resulting in there being severallevels (for example, five levels) of light reflectivity for thereproduction laser beam. Accordingly, by associating the respectivelight reflectivity levels with respective data contents, it is possibleto record one out of a plurality of data in one virtual recording cell.Here, the expression “light transmissivity” refers to the proportion ofthe reproduction laser beam irradiated onto a virtual recording cellthat passes through the virtual recording cell, and the expression“light reflectivity” refers to the proportion of the reproduction laserbeam irradiated onto a virtual recording cell that passes through thevirtual recording cell, is reflected off a reflective layer of themulti-level optical recording medium, and then passes back through thevirtual recording cell and exits to the periphery of the multi-leveloptical recording medium.

[0004] When reproducing recording data recorded on this multi-leveloptical recording medium (hereinafter this is also referred to as“multi-level reproduction”), the recording data is reproduced byspecifying the light reflectivity of a virtual recording cell irradiatedwith the reproduction laser beam as a first, second, third, etc., levelout of a plurality of light reflectivity levels. Here, in order torecord recording data (recording marks M) at an even higher density thana normal binary optical recording medium, the size of the virtualrecording marks S on a multi-level optical recording medium is set so asto be smaller than the pits on a binary optical recording medium. Also,the reading speed (i.e., read number of recording marks) per unitreproduction time should be set higher for a multi-level opticalrecording medium than a normal binary optical recording medium and thestandard rotational speed (that is, a linear velocity when recording andreproduction are carried out at 1× speed) is set higher than thestandard rotational speed of a normal binary optical recording medium.This means that during multi-level reproduction, in the case where theirradiation power of a reproduction laser beam is set at a similar levelas the irradiation power (in this case, 0.7 mW or below) of thereproduction laser beam set for a normal binary optical recordingmedium, the ratio (“C/N”, also referred to below as “SNR”) of a noisecomponent included in the reflected light increases, which can lead to adecrease in the read accuracy for recording data and to the occurrenceof tracking errors when the medium is rotated at high speed.Accordingly, the present inventors tried to solve the above problems byincreasing the irradiation power of the reproduction laser beam duringmulti-level reproduction.

DISCLOSURE OF THE INVENTION

[0005] The present inventors discovered the following improvement for areproduction method for a multi-level optical recording medium. That is,in this reproduction method, the emission power of the reproductionlaser beam should be set at a value that exceeds the emission power of areproduction laser beam used when reproducing a binary optical recordingmedium to prevent decreases in the read accuracy and the occurrence oftracking errors and the like. However, when a reproduction laser beamwith a high output is repeatedly irradiated onto a multi-level opticalrecording medium, the recording layer of the multi-level opticalrecording medium is altered by the irradiation of the reproduction laserbeam and the signal level falls, resulting in a reduction in the SNR(hereinafter this phenomenon is called “reproduction deterioration”) andin the risk that the recording data will not be reproduced correctly.When the emission power of the reproduction laser beam is set low,reproduction deterioration is avoided, but there can be a fall in readaccuracy as described above. However, for multi-level optical recordingmedia, a range of emission power for a reproduction laser beam that canimprove read accuracy while also suppressing reproduction deteriorationto a level where deterioration effectively does not occur has not yetbeen ascertained, with it being preferable to improve this situation.

[0006] The present invention was conceived in view of the abovesituation, and it is a principal object of the present invention toprovide a reproduction method and reproduction apparatus for amulti-level optical recording medium that can improve read accuracy forrecording data while also suppressing reproduction deterioration of themulti-level optical recording medium to a level where deteriorationeffectively does not occur.

[0007] A reproduction method for a multi-level optical recording mediumaccording to the present invention is a method of reproducing amulti-level optical recording medium on which recording data has beenrecorded according to a multi-level recording method in which severallevels are defined for light reflectivity of recorded parts, thereproduction method rotating the multi-level optical recording medium,emitting a reproduction laser beam towards the multi-level opticalrecording medium, and reproducing the recording data based on anelectric signal generated in accordance with a received level ofreflected light, wherein an emission power of the reproduction laserbeam is set in a range of 1.0 mW to 2.5 mW inclusive when themulti-level optical recording medium is rotated at a linear velocity ina range of 9 m/s to 25 m/s inclusive.

[0008] A reproduction apparatus according to the present inventionreproduces the recording data in accordance with the above-describedreproduction method for a multi-level optical recording medium andincludes: a laser emitting unit that emits the reproduction laser beam;a laser receiving unit that receives the reflected light emitted fromthe laser emitting unit and reflected by the multi-level opticalrecording medium and also outputs the electric signal in accordance withthe received level of the reflected light; and a control unit thatcontrols the emission power of the reproduction laser beam emitted fromthe laser emitting unit, wherein the control unit controls the laseremitting unit so that the emission power of the reproduction laser beamis in the range of 1.0 mW to 2.5 mW inclusive when the multi-leveloptical recording medium is rotated at a linear velocity in the range of9 m/s to 25 m/s inclusive.

[0009] The above reproduction method and reproduction apparatus for amulti-level optical recording medium set the emission power of thereproduction laser beam in the range of 1.0 mW to 2.5 mW inclusive whenthe multi-level optical recording medium is rotated at a linear velocityin the range of 9 m/s to 25 m/s inclusive, so that the SER and BER canbe maintained at levels that do not hinder the reading of the recordingdata, resulting in an improvement to the read accuracy for the recordingdata, and the reproduction deterioration of the multi-level opticalrecording medium can be suppressed to a level at which deteriorationeffectively does not occur, even after continuous reproduction.

[0010] It should be noted that the disclosure of the present inventionrelates to a content of Japanese Patent Application 2001-275911 that wasfiled on 12 Sep. 2001 and the entire content of which is hereinincorporated by reference.

BRIEF DESCRIPTION OF DRAWINGS

[0011]FIG. 1 is a perspective view, in which one part has been cut awayto show the construction of an optical recording medium 10 according toan embodiment of the present invention.

[0012]FIG. 2 is a schematic view showing recording marks Ma to Mgrecorded on the optical recording medium 10.

[0013]FIG. 3 is a block diagram showing the construction of arecording/reproduction apparatus 1.

[0014]FIG. 4 is a characteristics graph showing the relationship betweenan emission power of an emitted laser La and an SER.

[0015]FIG. 5 is a characteristics graph showing the relationship betweenthe emission power of the emitted laser La and a BER.

[0016]FIG. 6 is a characteristics graph showing the relationship betweenthe emission power of the emitted laser La and an SNR.

[0017]FIG. 7 is a table useful in explaining the relationship betweenthe emission power of the emitted laser La, the rotational speed (linearvelocity) of the optical recording medium 10, an SER, SNR and first readaccuracy measured during a first reproduction, and a SER and SNRmeasured after one million consecutive reproductions, and the readaccuracy measured after continuous reproductions.

BEST MODE FOR CARRYING OUT THE INVENTION

[0018] Hereafter, a preferred embodiment of a reproduction method andreproduction apparatus for a multi-level optical recording mediumaccording to the present invention will be described with reference tothe attached drawings.

[0019] First, a construction of a multi-level optical recording medium10 according to the present invention (also referred to as the “opticalrecording medium 10”) will be described with reference to FIG. 1.

[0020] The multi-level optical recording medium 10 is a CD-R typeoptical recording medium (recordable optical recording medium) and asshown in FIG. 1, includes a substrate 11, a recording layer 12, areflective film 13, and a protective layer 14. The substrate 11 isformed of transparent resin in the shape of a disc through which areproduction laser beam passes before the beam reaches the recordinglayer 12. In one surface of the substrate 11 (the upper surface in FIG.1), grooves 11 a for guiding a laser beam and lands 11 b are formed inspirals from a central periphery of the substrate 11 to an outerperiphery.

[0021] The recording layer 12 is formed using an organic dye, such ascyanine, merocyanine, a methine dye and derivatives of the same, abenzenethiol metal complex, a phthalocyanine dye, a naphthalocyaninedye, and azo dye, and is formed by such organic dye being applied so asto cover the grooves 11 a and the lands 11 b. The recording layer 12decomposes and is altered when a recording laser beam is irradiated by arecording apparatus, with the light transmissivity of the recordinglayer 12 changing according to the irradiated amount of the laser beam.The reflective film 13 is a thin layer for reflecting the reproductionlaser beam that has passed through the substrate 11 and the recordinglayer 12 during the reproduction of the recording data recorded on theoptical recording medium 10, and is formed with a metal such as gold andsilver as a main material by sputtering, for example, on the recordinglayer 12. The protective layer 14 is a layer that protects thereflective film 13 and the recording layer 12 and is formed so as tocover an outer surface of the reflective film 13.

[0022] Next, the basic principles of recording on the optical recordingmedium 10 will be described with reference to FIGS. 1 and 2.

[0023] On the multi-level optical recording medium 10, as shown in FIG.1, virtual recording cells S, S, . . . are (virtually) set as recordingunits by virtually dividing the grooves 11 a in the direction ofrotation of the optical recording medium 10 (the circumferentialdirection). Here, as shown in FIG. 2, the length of the virtualrecording cells S in the direction along the grooves 11 a is set shorter(0.6 μm, for example) than the focused beam diameter (the beam waistdiameter) D (1.5 μm, for example). In this case, although it is possibleto freely select the width of the grooves 11 a and the track pitch ofthe optical recording medium 10, as shown in FIG. 2, the unit width ofthe virtual recording cells S for the present optical recording medium10 is set so as to be equal to or slightly narrower (0.6 μm, forexample) than the width of the grooves 11 a. Accordingly, the virtualrecording cells S can be efficiently arranged with respect to the radialdirection of the optical recording medium 10. It should be noted thatthe virtual recording cells S are first and foremost virtual cells and,rather than actually existing as the rectangular shapes shown in FIG. 2,are imagined by a recording/reproduction apparatus during signalprocessing as part of multi-level recording/reproduction.

[0024] In this case, by controlling the irradiation time of a recordinglaser beam emitted from the pickup of a recording apparatus (that is,the “irradiated amount” of a laser beam) at a number of levels accordingto a value of the recording data, as shown in FIG. 2 recording marks Mato Mg (also referred to in general as the recording marks M when nodemarcation is required) with different degrees ofdecomposition/alteration for (mainly the organic dye of) the recordinglayer 12 are formed in the virtual recording cells S. It should be notedthat the degree of decomposition/alteration has been schematicallyillustrated by the sizes of the recording marks M. Also, the virtualrecording cells S in which the recording marks M have been formed formthe recording parts of the recording layer 12. When the recording ofrecording data is carried out by a recording laser beam, the opticalrecording medium 10 is rotated while being irradiated with the recordinglaser beam, so that the recording marks M are formed as ovals whoselength depends on the irradiation time.

[0025] Also, when multi-level recording of recording data is carried outon the optical recording medium 10, the respective degrees ofdecomposition/alteration (i.e., the change in the light transmissivity)of the recording marks Ma to Mg are set so that light reflectivity whena reproduction laser beam is irradiated onto a part including thevirtual recording cells S has seven levels (eight levels when anon-recorded part is included), for example. In this case, the smallerthe degree of decomposition/alteration for the recording layer 12, thelarger the light reflectivity. This means that virtual recording cells Sin which a recording mark M has not been recorded characteristicallyhave the highest light reflectivity, and parts including the virtualrecording cells S in which the smallest recording marks Ma have beenformed have the highest light reflectivity out of the recording marks M,with the light reflectivity decreasing in order for parts including thevirtual recording cells S in which the recording marks Mb to Mfrespectively have been formed and parts including a virtual recordingcell S in which the largest recording mark Mg has been formed having thelowest light reflectivity.

[0026] In this case, the expression a “part including virtual recordingcells S” refers to an irradiated part of the recording layer of theoptical recording medium 10 where the recording or reproduction laserbeam has been focused. Accordingly, this expression does not apply to anumber of virtual recording cells S.

[0027] That is, depending on the focused beam diameter of the recordingor reproduction laser beam, the irradiated part where the laser beam hasbeen focused can include a certain part of a single virtual recordingcell S and a part that includes one or more virtual recording cells S.

[0028] This means that by controlling the irradiated amount of therecording laser beam at the part including the virtual recording cell Sand appropriately setting the area ratio of the decomposed/altered part(that is, the light reflectivity of the recording layer 12), it ispossible to form the recording marks Ma to Mg that have seven levels oflight reflectivity when the reproduction laser beam is irradiated.

[0029] Next, the recording of recording data on the optical recordingmedium 10 and the recording/reproduction apparatus 1 that carries outthe reproduction of recording data will be described with reference toFIG. 3.

[0030] The recording/reproduction apparatus 1 corresponds to thereproduction apparatus according to the present invention, and isconstructed so as to be able to reproduce recording data according tothe method of reproducing a multi-level optical recording mediumaccording to the present invention. The recording/reproduction apparatus1 is a so-called CD-R recorder, and includes a spindle motor 2, a pickup3, a spindle servo 4, a feed servo 5, a focus tracking servo 6, and acontrol unit 7. In this case, driving of the spindle motor 2 iscontrolled by the spindle servo 4 as described later, so that theoptical recording medium 10 is rotated at a fixed linear velocity.

[0031] The pickup 3 is constructed so as to be integrated with a laseremitting section and a laser receiving section in the present invention,a laser is driven by a laser driver (neither is shown) under the controlof the control unit 7 so that a recording laser beam or reproductionlaser beam (emitted laser La) is emitted onto the optical recordingmedium 10. By doing so, the recording of the recording marks M in thevirtual recording cells S and the output of an electric signal inaccordance with a level of a reflected laser Lb reflected by the virtualrecording cells S in which the recording marks M are recorded arecarried out. In this case, during the reproduction of recording data,the laser driver of the pickup 3 adjusts, under the control of thecontrol unit 7, the emission power of the emitted laser La in a rangeof 1. mW to 2.5 mW inclusive in accordance with the rotational speed ofthe optical recording medium 10. The pickup 3 is equipped with anobjective lens and half mirror (neither is shown), and focuses a laserbeam for recording or reproduction on the recording layer 12 of theoptical recording medium 10. More specifically, the objective lens issubjected to focus tracking control by the focus tracking servo 6,resulting in the laser beam for recording or reproduction being focusedon the recording layer 12 of the optical recording medium 10. The pickup3 is moved reciprocally in a radial direction of the optical recordingmedium 10 by the feed servo 5 between an inner periphery and an outerperiphery of the optical recording medium 10.

[0032] Under control of the control unit 7, the spindle servo 4 controlsthe rotation of the spindle motor 2 so that the spindle motor 2 rotatesat a designated fixed speed within a range of linear velocities of 9 m/sto 25 m/s inclusive. The control unit 7 controls the driving of thepickup 3, the spindle servo 4, the feed servo 5, and the focus trackingservo 6, and based on an electric signal outputted from the pickup 3,interprets the recording data recorded on the recording layer 12.

[0033] Next, the method of reproducing recording data whereby therecording/reproduction apparatus 1 reproduces recording data recorded onthe optical recording medium 10 will be described.

[0034] When the optical recording medium 10 has been loaded into therecording/reproduction apparatus 1, the control unit 7 controls thespindle servo 4 to drive the spindle motor 2 so that the opticalrecording medium 10 is rotated at a linear velocity of 9 m/s, forexample. At the same time, the control unit 7 drives the feed servo 5 tomove the pickup 3 to a lead-in area. Next, the control unit 7 causes thepickup 3 to emit the emitted laser La and also drives the focus trackingservo 6 to carry out focus tracking control of the objective lens of thepickup 3. At this time, the control unit 7 carries out driving controlof the pickup 3 so that the emission power of the emitted laser La is1.1 mW, for example. By doing so, the emitted laser La emitted by thepickup 3 is irradiated onto the lead-in area of the optical recordingmedium 10, and the reflected laser Lb is received by the pickup 3. Atthis time, the pickup 3 outputs an electric signal in accordance withvarious information recorded using wobble, for example, in the lead-inarea, so that the control unit 7 can determine, based on the electricsignal, whether the optical recording medium 10 is a multi-level opticalrecording medium.

[0035] After this, the control unit 7 carries out driving control of thespindle servo 4 and while keeping the rotational speed of the opticalrecording medium 10 at a fixed linear velocity of 9 m/s, the controlunit 7 carries out driving control of the feed servo 5 so that thepickup 3 is moved to a position above an innermost groove 11 a. Thecontrol unit 7 drives and controls the pickup 3 so that the emissionpower of the emitted laser La is maintained at 1.1 mW. By doing so, theemitted laser La emitted from the pickup 3 is irradiated onto thevirtual recording cells S, S. . . . inside the grooves 11 a on theoptical recording medium 10, and the reflected laser Lb reflected by thereflective film 13 is received by the pickup 3. At this time, the pickup3 outputs an electric signal in accordance with the light reflectivityof the recording marks Ma to Mg recorded in the virtual recording cellsS, S, . . . , and based on this electric signal, the control unit 7 caninterpret the recording data recorded on the optical recording medium10. As a result, the recording data is reproduced.

[0036] Next, the relationship between the emission power of the emittedlaser La and the read accuracy for the recording data/reproductiondeterioration will be described with reference to FIGS. 4 to 7.

[0037] When each of various types of optical recording media 10 withrecording layers 12 of different materials was rotated at a fixed speed,for example, at a linear velocity of 9 m/s, and the emission power ofthe emitted laser La was increased in stages, it was confirmed that theread accuracy of the recording data increases for the respective opticalrecording media 10 approximately in proportion to the emission power ofthe emitted laser La.

[0038] In this case, as shown in FIG. 4, when the emission power of theemitted laser La is set at 1.0 mW or above, the SER (the ratio of readerrors to correct data) can be maintained at a level (4% or below) thatdoes not hinder the reading of the recording data. It should be notedthat in FIG. 4, respective SER characteristics corresponding to therespective optical recording media 10 on which the recording layers 12are formed of different materials are shown by solid lines or brokenlines, in FIG. 5, respective BER characteristics corresponding to therespective optical recording media 10 on which the recording layers 12are formed of different materials are shown by solid lines or brokenlines, and in FIG. 6, respective SNR characteristics corresponding tothe respective optical recording media 10 on which the recording layers12 are formed of different materials are shown by solid lines or brokenlines. More specifically, in FIGS. 4 to 6, the optical recording medium10 shown by the solid line A is constructed with the recording layer 12made of cyanine dye, the optical recording medium 10 shown by the brokenline B is constructed with the recording layer 12 made of phthalocyaninedye including Zn and V as the metal center, and the optical recordingmedium 10 shown by the broken line C is constructed with the recordinglayer 12 made of phthalocyanine dye including Pd as the metal center.When, as shown in FIG. 5, the emission power of the emitted laser La isset at 1.0 mW or above, the BER (bit error rate) is kept at a level(1×10⁻³ or below) that does not hinder the reading of the recordingdata. This is because as shown in FIG. 6, when the emission power of theemitted laser La is set at 1.0 mW or above, the ratio of the noisecomponent (the SNR) for the reflected laser Lb can be kept at a level(28.6 dB or above) that does not hinder the reading of the recordingdata. In this case, even if the optical recording medium 10 is rotatedat a fixed linear velocity of 25 m/s, if the emission power of theemitted laser La is set at 1.0 mW or above, in the same way as when theoptical recording medium 10 is rotated at a linear velocity of 9 m/s, itwas confirmed that the SER and BER can be maintained at levels that donot hinder the reading of the recording data.

[0039] On the other hand, as shown in FIGS. 4 to 6, when the emissionpower of the emitted laser La is successively reduced from 1.0 mw, theSER and BER gradually increase and the SNR gradually decreases, so thatdue to the type of the material of the recording layer 12 and unevennessduring manufacturing, there is increased probability of the SER, BER,and SNR moving away from the levels described above that do not hinderthe reading of the recording data.

[0040] In reality, as shown in FIG. 7, for an optical recording medium10 constructed with a recording layer 12 made of phthalocyanine dyeincluding Pd as the metal center, it was confirmed that when theemission power of the emitted laser La is set at 0.9 mW that is below1.0 mW, the first read accuracy of the recording data and the readaccuracy after continuous reproduction were both below a level at whichthe reading of the recording data is hindered. On the other hand, it wasconfirmed that when the emission power of the emitted laser La is set ina range of 1.0 mW to 2.5 mW inclusive, it was actually possible tocorrectly read the recording data on the optical recording medium 10.Also, as shown in FIG. 7, when the emission power of the emitted laserLa is set at 2.6 mW, it was confirmed that although the first readaccuracy of the recording data is at a level that does not hinder thereading, the emitted amount of the emitted laser La per unit time forthe recording layer 12 becomes large, more so at a linear velocity of 9m/s than a linear velocity of 25 m/s, which causes alteration to therecording layer 12, so that the read accuracy after continuous readingfalls, which is to say, reproduction deterioration occurs.

[0041] Accordingly, it was understood that when the optical recordingmedium 10 is rotated at a linear velocity in a range of 9 m/s to 25 m/sinclusive, by setting the emission power of the emitted laser La in arange of 1.0 mW to 2.5 mW inclusive, it is possible to correctlyreproduce the recording data while suppressing reproductiondeterioration to a level where deterioration effectively does not occur.

[0042] It should be noted that the ratings in the “first reproductionaccuracy” and the “read accuracy after continuous reproduction” columnsshown in FIG. 7 were produced by the following evaluation method for therespective SER and SNR values measured during a first reproduction andafter one million reproductions. In this evaluation method, based on thecharacteristic that read accuracy decreases as the value of the SERincreases, zero points were given for an SER of 4.2% or above, one pointwas given for an SER of 4.0% or above but below 4.2%, and two pointswere given for an SER of below 4.0%, based on the characteristic thatread accuracy decreases as the value of the SNR falls, zero points weregiven for an SNR of 28.5 dB or below, one point was given for an SNR ofabove 28.5 dB but no greater than 28.9 dB, and two points were given foran SNR of above 28.9 dB, and the read accuracy was then judged based onthe overall magnitude of the total of the points for the SER and SNR.More specifically, when the points total is zero points, it isdetermined that there is a high risk of difficulties in reading theoptical recording medium 10 correctly (indicated by the code “×”).

[0043] Similarly, when the points total is one point, it is determinedthat there will effectively be no problems (indicated by the code “Δ”).Also, when the points total is two or three points, it is determinedthat the recording data can be read sufficiently (indicated by the code“◯”). When the points total is four points, it is determined that theoptical recording medium 10 will definitely be read correctly (indicatedby the code “⊚”).

[0044] It should be noted that the present invention is not limited tothe embodiment described above and that various modifications arepossible as appropriate. For example, in the above embodiment, anexample where the emission power of the emitted laser La is set at 1.1mW when the optical recording medium 10 is rotated at a linear velocityof 9 m/s is described, but the present invention is not limited to this.For example, when the optical recording medium 10 is rotated at a linearvelocity of 9 m/s or below, the range of emission power (i.e., a lowerlimit of 1.0 mW and a higher limit of 2.5 mW) of the reproduction laserbeam can be reduced in proportion to the linear velocity, and when theoptical recording medium 10 is rotated at a linear velocity of above 25m/s, the range of emission power (from a lower limit to an upper limit)of the reproduction laser beam can be increased in proportion to thelinear velocity, so that it is possible to correctly reproduce therecording data while suppressing reproduction deterioration of amulti-level optical recording medium to a level where deteriorationeffectively does not occur.

INDUSTRIAL APPLICABILITY

[0045] As described above, according to the method of reproducing amulti-level optical recording medium and reproduction apparatusaccording to the present invention, by setting the emission power of areproduction laser beam in a range of 1.0 mW to 2.5 mW inclusive when amulti-level optical recording medium is rotated at a linear velocity ina range of 9 m/s to 25 m/s inclusive, it is possible to keep the SER andBER at levels that do not hinder the reading of the recording data. Thismeans that it is possible to realize a reproduction method andreproduction apparatus for a multi-level optical recording medium thatcan increase the read accuracy for recording data and can suppress thereproduction deterioration of the multi-level optical recording mediumto a level where deterioration effectively does not occur, even whencontinuous reproduction is carried out.

1. A reproduction method for a multi-level optical recording medium onwhich recording data has been recorded according to a multi-levelrecording method in which several levels are defined for lightreflectivity of recorded parts, the reproduction method rotating themulti-level optical recording medium, emitting a reproduction laser beamtowards the multi-level optical recording medium, and reproducing therecording data based on an electric signal generated in accordance witha received level of reflected light, wherein an emission power of thereproduction laser beam is set in a range of 1.0 mW to 2.5 mW inclusivewhen the multi-level optical recording medium is rotated at a linearvelocity in a range of 9 m/s to 25 m/s inclusive.
 2. A reproductionapparatus that reproduces the recording data in accordance with thereproduction method for a multi-level optical recording medium accordingto claim 1, comprising: a laser emitting unit that emits thereproduction laser beam; a laser receiving unit that receives thereflected light emitted from the laser emitting unit and reflected bythe multi-level optical recording medium and also outputs the electricsignal in accordance with the received level of the reflected light; anda control unit that controls the emission power of the reproductionlaser beam emitted from the laser emitting unit, wherein the controlunit controls the laser emitting unit so that the emission power of thereproduction laser beam is in the range of 1.0 mW to 2.5 mW inclusivewhen the multi-level optical recording medium is rotated at a linearvelocity in the range of 9 m/s to 25 m/s inclusive.